Process of simultaneously purifying and dehydrating caustic alkali solutions containing chlorates



Sept. 9, 1952 D. J. PYE 2,610,105

PROCESS OF SIMULTANEOUSLY PURIFYING AND DEHYDRATING CAUSTIC ALKALI SOLUTIONS CONTAINING CHLORATES Filed March 26, 1951 INVENTQR. 04 via J. Pye

A TTORNE Y$ Patented .Sept. 9, 1952 OFFICE PROCESS OF SIMULTANEOUSLY PURIFYIN G AND DEHYDRATING C AUSTIC ALKALI SO- LUTIONS CONTAINING CHLORATES David J. Pye, Concord, Calif., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware 1 Application March 26, 1951, Serial No.'217,658

where alkali metal chloride contained in the 7 same is separated by crystallization. The partially concentratedliquor freed from salt is then further evaporated; until 1 completely dehydrated. Thus, in the long-established practice for manufacture of solid caustic soda, the dilute ,cell liquor is first evaporated in salting-out evaporators under vacuum, to; a concentration of about 50 per cent NaOH, whereat the solution is substantially freed of salt The 50 per cent liquor separatedfrom the salt is then transferred to directfired pots and boiled down to a final finishing temperature of about 440 C. to complete the dehydration.

The electrolyticcell liquors normally contain small-but significant amounts of chlorate, which is not, removed with the salt on evaporation but remains dissolved in the concentrated solution. The amounts of chlorate found in the cell liquors can vary considerably with the particular type of electrolytic cell and its manner of operation,

but may be from as low as 500 to as much as.

5000 or more parts per million (p. p. m.), based on the solids content of the liquor. The-pres ence of chlorate injthe concentrated liquors is highly objeotionablein causing corrosion of the evaporators. The problem of corrosion due to chlorate isnot serious in the evaporation "of dilute caustic alkalijiquors at the moderate jteni'a peratures employed, butat the higher temperatures required for concentrating strong liquors corrosion due to chlorate occurs at a rate increasing rapidly with rise of temperature. T

An especial problem has been created by the more recent practice of concentrating caustic alkali liquors in vacuum evaporators to much higher strengths than was the case in the older practice. Thus, it is now well-known practice to concentrate the de-salted 50 per cent caustic soda liquors in vacuum evaporators 1 up to a strength of about 70 per cent NaOH, and to use such high strength liquor forfeeding the direct- At the higher concentrafired finishing pots.

3 Claims. (01. 23-184) tions and temperatures involved in this practice, nickel has been used instead of steel for those parts of the evaporators which are in contact with the hot concentrated liquors, on account of the greater resistance ofnickel to chemical attack by the hot alkali. While nickel is resistant to the action of alkali, it is attacked and corroded by chlorates present in the concentrated solutions to a degree influenced by the temperature, chlorate concentration, time of contact and extent of nickel surface. It is, therefore, essential to remove chlorates completely from caustic alkali liquors which are processed by high temperature evaporation of concentrated liquors in nickel equipment.

Various methods have been proposed for purifying electrolytic caustic alkali liquors from chlorates, but noneof them heretofore has been wholly satisfactory in practice, either for failure to remove chlorates completely or because they introduced other objectionable impurities into the caustic alkali product. Another objection has been that such methods called for a separate treatment'of the liquor and removal, by filtration, sedimentation, or otherwise, of excess treating agent orof precipitated solids formed in the treatment. Prior purification methods also depended upon reactions requiring substantial time for completion, thus necessitating the provision of extra equipment for larger inventory capacity than would otherwise be required.

Itis, therefore, a principal object of the invention to provide a method for the removal of chlorates from concentrated electrolytic caustic alkali liquors which avoids the aforementioned disadvantages of prior methods. A particular object is to provide a method which is carried out simultaneously with the high temperature evaporation of the liquors, requiring a minimum of extra equipment. More particularly, it is an object of the invention to enable electrolytic caustioalkali liquors to beconcentrated to high degree, .even topractically complete dehydration, by continuous evaporation in nickel equipment, without substantial corrosion of the equipment or pickup of nickel impurity in the product.

I have found that common cane or beet sugar, sucrose, is an exceedingly effective agent for reducing chlorates in strong causticalkali liquors at elevated temperatures of the order required for evaporation of such strong liquors. The

' reduced pressure.

- 3 moval of chlorate by treatment with sucrose can, therefore, be carried out in the high temperature evaporator simultaneously with the evaporation of the strong liquor.

It has thus been made practically possible to conduct the continuous concentration of electrolytic caustic alkali liquors by evaporation in a tubular evaporator having nickel surfaces exposed to the liquors, so as not only to obtain a product having a strength greater than 70 per cent, which heretofore has been the practical limit, but even to carry the evaporation further to the point of substantially complete dehydration so as to obtain a molten product. The present improvements constituting the invention are fully described in the following specification and annexed drawing.

In said drawing, the single figure is a diagrammatic flow sheet illustrating a preferred embodiment of the invention.

According to the invention, caustic alkali liquors containing chlorates as impurity are treated with sucrose, which is intimately incorporated therewith by solution or dispersion, and heated at a temperature sufliciently high to cause rapid reduction or decomposition of the chlorate by chemical reaction with the sucrose. Such chemical reduction may take lace at a moderate but substantial rate at a temperature as low as 150 C., but for practical operation temperatures materially higher are desirable. The reaction rate increases with rise of temperature until atfabout' 200 to 220 C. the reduction of chlorate in the caustic liquor is practically instantaneous. rate is reduced at temperatures above 200 C.

It follows that the caustic alkali liquors to be purified from chlorate accordingto the invention should be of suflicient concentration to permit heatin them above 150 C., e. g. to 200 C. or higher, at superatmospheric, atmospheric or That is to say, their boiling points should be above such temperature at the pressure employed. Generally speaking, caustic soda or caustic potash solutions adapted for purification according to the invention have a concentration of 50 per cent by weight, or more. Such liquors containing the usual amounts of chlorate can be heated in nickel equipment to temperatures as high as 150 C. substantially 'without corrosive attack on the metal by the chlorate, provided the contact time is not excessive, but at higher temperatures the rate of attack soon becomes appreciable and increases with the temperature. In nickel evaporators ordinarily used for concentrating such strong liquors, corrosion of the nickelby the chlorate in the liquor become rapid and severe at'about 280 C.

Prior to treatment for removal of chlorate accordingf'to the invention, the caustic alkali. liquor may be evaporated under suitable pressure by'usual means to any degree of concentation that may be attained at temperatures below 150 C. To such preconcentrated liquor is then added a sufiicient amount of sucrose to reduce completely the chlorate contained in the liquor, and the evaporation is continued in nickel equipment at progressively higher temperatures until dehydration is largely or substantially complete, at which point the alkali is said to be finished. During such high temperature evaporation the added sucrose completely reduces the chlorate in the liquor so rapidly as to prevent corrosive attack on the nic el S rfaces For best results the ch1o of the evaporator which are in contact with the liquor. The ease and simplicity of the treatment is apparent, particularly inthe fact that no separate treatment of the liquor outside of the evaporator is required for removal of chlorate.

Thus, the new method may be readily adapted to any existing commercial practice for concentrating caustic alkali liquors by the addition of equipment for proportioning and feeding the sucrose to the liquor at an appropriate stage in its concentration.

As an example, in concentrating the previously (re-salted 50 per cent caustic soda liquor under vacuum in a nickel-lined evaporator to a concentration of about 70 per cent NaOH, if the evaporator is operated under avacuum of about 22 inches Hg or more, concentration up to '10 per cent NaOI-I takes place at temperatures below C. Up to this point corrosion of the evaporator due to the presence of chlorate in the liquor is negligible. The sucrose may then be added to the 70 'per cent liquor in amount suiiicient to reduce the chlorate content thereof, and the evaporation continued at higher temperatures up to substantially complete dehydration at a temperature above the melting point of the solid caustic alkali. At such higher temperatures reduction of chlorate takes place so rapidly that there is substantially no corrosion of the evaporator or take-up of nickel impurities by the caustic alkali. V

It is not necessary, however, to add the sucrose to the caustic soda solution at any particular point in the various stages of concentration of the same to the completely dehydrated state, provided that the addition is made before the temperature of evaporation reaches the point where corrosion of the nickel evaporator is to be feared or expected. 7

The stoichiometrical relationship of sucrose to chlorate in the present process has not been precisely ascertained. The reduction of chlorate at the expense of the sucrose results in ultimate decomposition of the latter to CO2 and H20, while the chlorate is reduced to chloride. The CO2 in the presence of caustic alkali is, of course, converted to alkali metal carbonate. Both the chloride and carbonate are usual impurities in caustic alkali, which within the limits of commercial specifications are unobjectionable.

Suclrlimits are not normally exceeded in the practice of this invention with liquors of usual chlorate content. The stoichiometrical propor tions may be tentatively stated according to the equation:

While the equation may not show exactly the nature of the chemical reactions involved here- 1n, it expresses the over-all result. According to. the above equation, approximately 0.4 part by weight of sucrose is theoretically required for the reduction of 1 part of NaClOx.

In practice, somewhat more than the theoretical amount of sucrose should be used. Apparently decomposition of the sucrose by action of the hot strong caustic alkali takes place to some extentsimultaneously with the reaction of sucrose with the chlorate. Thus, for complete, reduction of the chlorate about 1.5 to 2.5 parts of sucrose per part of chlorate are required, although larger amounts may be used. For example, if the liquor contains 2000 p. p. m. of chlorate, dry basis, calculated at NaClOs, ie 4 pounds per ton, about 8 or 9 pounds of sucrose per ton may be added to provide a safe margin to insure complete reduction of the chlorate. It is well not to use a larger excess of sucrose than experience has shown to be necessary, in order to avoid too great build-up of carbonate in the product. As stated, the sucrose consumed in reducing the chlorate is converted to an equivalent amount of carbonate, while the excess of sucrose is decomposed to H20 and CO or, to some extentg CO2 under the conditions of treatment, such additional 002 also appearing as carbonate in the product. r I

' A preferred procedure for concentrating caustic soda solutions and simultaneously removing chlorate therefrom according to the invention is illustrated by the flow diagram shown in the drawing. A 50 per cent NaOH liquor is fed through pipe I into a vacuum evaporator 2, the exhaust vapors from which are taken off through pipe 3 connected to a source of vacuum (not shown). In evaporator 2 the liquor is concentrated to about '70 per cent NaOH at atemperature below 150 C. The 70 per cent liquor isremoved from the evaporator through circulating pipe 4 by means of pump 5 and forwarded through pipe Ii to storage tank I provided with steam coils or other source of heat to hold the contents thereof liquid and preheat the same, if necessary, to a temperature between about 100 C. and the boiling point of the solution. A supply tank 8 contains a solution of sugar (sucrose), which is fed into tank I through line 9 in proportion required for reduction of the chlorates in the liquor. Suitable mixing apparatus may be installed in tank I, if required, for intermixing the sugar solution with the caustic soda solution, and metering controls may be included in line 9 for regulating the amount of sugar solution to be added to tank I. The '70 per cent caustic soda liquor containing the added sugar is pumped continuously through pipe II] to the base of a finishing evaporator I I. Evaporator II is an elongated single-pass vertical tubular heat-exchanger of usual design, in which the tubes and other parts in contact with the caustic liquor are made of nickel. It is heated by the vapors of a high-temperature heat transfer agent generated in boiler I2 and circulated through the evaporator by means of vapor pipe I3 and return pipe I4. A suitable high-temperature heat transfer agent is known by the tradename of Dowtherm, which consists of an eutectic mixture of diphenyl and diphenyloxide as described in U. S. Patent No. 1,882,809,

.having an atmospheric boiling point of 258 C.

The liquor in evaporator II is evaporated, preferably under reduced pressure, to substantially complete dehydration, being supplied to the tubes of the evaporator at a flow rate in proportion to the heat input providing a sufiicient residence time, on the order of a few seconds to about 2 minutes, for substantially complete evaporation of water, and is discharged through exit pipe I5 as an entrained mixture of molten caustic soda and water vapor into a separator I6. The molten caustic soda passes from separator I6 through pipe II to a heated holding pot I8, serving as an inventory tank in which the molten material is held until withdrawn, as indicated by line I0, to a drum-filling station or a flaking machine, or the equivalent. The separated water vapor passes through vapor pipe 20 to barometric jet condenser 2I, which provides vacuum for evaporator II, the condensate discharging through leg 22.

For purpose of illustration, showing typical operating conditions of the process as described, evaporator 2 may be operated at a vacuum of 28 inches Hg, and the 70 per cent liquor is discharged from the evaporator at a temperature of about 115 C. In storage tank I the liquor is held at about 100 C., while being mixed with the sugar solution which is metered as a 10 per cent aqueous solution at the rate of about 8 to 9 pounds of sugar per ton of NaOH in the solution containing about 2000 p. p. m. (4 lbs/ton) of NaClOs. The treated liquor is fed to evaporator II at about 100 C. Evaporator II is heated with Dowtherm vapor supplied at about 365 to 380 C. and operated under vacuum of about 20 inches Hg at the top. With tubes inch 1. d. by 20 feet long, the feed liquor is immediately heated to boiling in the lower part of the tubes and flashes to an intimate liquidvapor mixture which is discharged at the top of the evaporator at a temperature of about 360 to 370 C. The molten caustic alkali separated in separator I6 contains per cent or more NaOH and is freefrom chlorate. Upon solidification it is substantially white and free from discoloration.

Reduction of chlorate in the feed liquor takes place practically instantaneously in the evaporator II, and is complete within the time during which the charged material passes through the evaporator. The molten caustic soda product is substantially free from nickel, containing at most amounts in the magnitude of 1 to 5 parts per million, corrosion of the nickel tubes or other parts of the evaporator being practically absent. Such corrosion would be so severe as to render the nickel tubes useless in a short time, unless the chlorate were removed, while the caustic soda product would be highly contaminated and discolored by nickel as impurity.

Instead of preconcentrating the 50 per cent NaOH liquor to 70 per cent, as described, the 50 per cent liquor may be fed directly to tank I for addition of sugar solution and preheating to a feed temperature of about to 0., and thence fed to evaporator I I. In such case the feed rate of the liquor to the evaporator would be adjusted to the heat input in the evaporator to carry the added evaporation load.

Although the preceding detailed description has been directed to the treatment and dehydration of caustic soda solutions, the method of treatment is the same for caustic potash solutions and similar results are obtained.

This application is a continuation-in-part of my prior application, Serial No. 33,907, filed June 18, 1948, now abandoned.

' I claim:

1. In the concentration of electrolytic caustic alkali solutions containing chlorate as impurity, wherein the electrolytic cell liquor is preconcentrated by evaporation to remove salt by crystallization, separated from salt crystals thereby formed and then further concentrated by evaporation, the improvement which consists in adding sucrose to such preconcentrated solution in amount in excess of 1 mol per 8 mols of chlorate contained therein and heating the so treated solution at temperatures above 200 C. to efiect further evaporation of the solution accompanied by chemical reduction of the chlorate.

2. A process of simultaneously dehydrating and purifying caustic alkali solutions containing chlorate as impurity which comprises preconcentrating the solution according to conventional 7* procedure by evaporation at temperatures below 150 0., adding sucrose to the preconcentrated solution in amount in excess of 1 mol per 8 mole of chlorate contained therein, continuously feeding the solution into an elongated tubular vaporizing zone maintained at a temperature above the melting point of the caustic alkali at a rate such that sufficient water is evaporated during passage through the zone to form substantially anhydrous molten caustic alkali, the chlorate being simultaneously reduced, and separating the molten caustic alkali from the aqueous vapors.

3. A process of dehydrating and simultaneously purifying caustic soda solutions containing chlorate as impurity which comprises adding sucrose to such solution having a concentration of about 50 to about 70 per cent NaOH, in amount sufficient for quantitative chemical reduction of the chlorate therein, continuously feeding the solution into an elongated tubular vaporizing zone maintained at a temperature above the melting point of anhydrous caustic soda at a rate such that the water content of the solution is flash-evaporated in a single pass through the zone, accompanied by chemical reduction of the chlorate, and separating the molten caustic soda from the aqueous vapor.

DAVID J. PETE.

8 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Textbook of Organic Chemistry, by G. M. Richter, 1938 ed., page 412. John Wiley and .Sons, Inc., N. Y.

Unit Spray-Drying and Reaction Chambers, bulletin No. 3, Garwood, N. J.

A Course in General Chemistry, by McPherson 8: Henderson, third ed., page 468. Ginn & 00., N. Y.

Hackhs Chemical Dictionary, revised by J. Grant, third ed, page 815. The Blakiston Co., Philadelphia, Pa.

1936, Bowen Research Corp 

1. IN THE CONCENTRATION OF ELECTROLYTIC CAUSTIC ALKALI SOLUTIONS CONTAINING CHLORATE AS IMPURITY, WHEREIN THE ELECTROLYTIC CELL LIQUOR IS PRECONCENTRATED BY EVAPORATION TO REMOVE SALT BY CRYSTALLIZATION, SEPARATED FROM SALT CRYSTALS THEREBY FORMED AND THEN FURTHER CONCENTRATED BY EVAPORATION, THEN IMPROVEMENT WHICH CONSISTS IN ADDING SUCROSE TO SUCH PRECONCENTRATED SOLUTION IN AMOUNT IN EXCESS OF 1 MOL PER 8 MOLS OF CHLORATE CONTAINED THEREIN AND HEATING THE SO TREATED SOLUTION AT TEMPERATURES ABOVE 200* C. TO EFFECT FURTHER EVAPORATION OF SOLUTION ACCOMPANIED BY CHEMICAL REDUCTION OF THE CHLORATE. 